Treatment of steam-boiler water



1 awnteu may ZU, 1950 UNITED STATES PATENT OFFICE RALPH E. HALL, OFPITTSBURGH, PENNSYLVANIA, ASSIGNOR TO JOHN M. HOPWOOD, 0F DORMONTBOROUGH, PENNSYLVANIA TREATMENT OF STEAM-BOILER WATER No Drawing.

This invention relates to the treatment of steam boiler Water, and moreparticularly to its treatment for the prevention of foaming. Foaming isa condition encountered in boilers, particularly Where the Watercontains soluble salts or saponifiable material, and consists in theformation of fine bubbles or foam in the steam space of the boiler. If asuflicient foam is formed, it is carried as entrained moisture throughthe steam outlet. This entrained moisture is highly objectionable in thesteam-utilizing devices, such as steam engines and turbines, and is alsovery objectionable in its action in a superheater.

hen steam is generated in a closed vessel, as a steam boiler, itsdelivery from the steam outlet devoid of droplets of boiler Water andconcomitant sludge, is a function among other things of thecharacteristics of the films of go the liquid phase enveloping theindividual steam bubbles, and the rate at which steam is delivered fromthe steam outlet. the films enveloping the bubbles initially at theevaporating surfaces coalesce readily to form larger bubbles and thesebubbles burst instantaneously at the steam-Water interface vof the steamdrum, no layer of foam can be in existence more than momentarily, andany tiny droplets of boiler Water, originating in the disruption of thefilm will be at a maximum distance from the steam outlet, and hence withminimum chance of being carried into the steam nozzle. But if the filmsdo not burst instantaneously, then the steam bubbles are pushed higherin the steam space by the ensuing bubbles, and the final disruption ofthe film may occur at any elevation in the steam space, or even be sodelayed that steam bubbles as such may enter the steam outlet from theboiler. Also, if a boiler is operating at low per cent of rating, it maydeliver dry steam, whereas at high per cent of rating the steam may beseriously contaminated by entrained moisture. This follows beis cause atlow per cent of rating, the available time for the disruption of thesteam bubble at a point in the steam space Where it will be harmless isgreater. Also, since the carrying power of a gas (steam) increases very50 rapidly with increase of velocity, at the high- Thus, if

Application filed December 4, 1926. Serial No. 152,730.

er per cent of rating, the steam will carry with it droplets of largersize, and therefore a larger fraction of all droplets formed, and thusincrease the amount of entrained moisture in the steam.

In order to insure the minimum of entrained moisture in the steam at thesteam outlet, therefore, it is necessary that the films of liquid phaseenveloping the individual bubbles of steam should readily coalesce; thattheir duration should be as nearly nil as possible at the steam-Waterinterface in the steam drum; and that any droplets formed in thebursting of the film should have the largest possible size.

The factors w hich influence the readiness of coalescenceoi" disruptionof the liquidphase films enveloping the steam bubbles have beenvariously described. The contamination of the boiler Water by oil hasfrequently been blamed for foaming. Likewise, foaming has been ascribedto a condition of carbonate or caustic alkalinity in the boiler Water,to the sodium sulfate and sodium chloride therein, to the presence ofsuspended matter, and to the combined effect of suspended and dissolvedsolids.

In an. extensive series of experiments conducted in experimental boilersin my laboratory, I have found that as successive additions 0 of sodiumsulphate, chloride, hydroxide, carbonate, or phosphate are made todistilled water in these boilers, the rate of coalescence and disruptionof the liquid films enveloping the bubbles at the evaporating surfacesand the steam-Water interface respectively steadily decreases, and thehead of foam just as surely increases. For all of these materials, thespecific effect under consideration was the same, but the amount ofsulfate, chloride, etc. expressed in parts per million necessary toproduce a like effect. Was different. Also, the presence of a slightamount of iron hydroxide such as might be caused by corrosion of theiron or steel of the boiler. or the presence of other precipitates ofsimilar character lessened the concentration of sulfate, chloride, etc.necessary to cause any chosen decrease in rate of film coalescence ordisruption. Greater amounts of precipitates of this type exercised asomewhat greater influence than the small amounts just described. Theeffect of oils was found to be closely connected with their compositionand the caustic alkalinity of the boiler water. Thus, an oil, heavilycompounded with a fatty material, in an alkaline water, produced a fargreater effect than one less heavily compounded, because of thesaponitication by the alkali in the boiler water of the fats or othersaponifiable material used on compounding the oil, and the consequentintroduction in solution into the boiler water of soap with itswell-recognized property of stabilizing liquid films against disruption.

Practically all natural waters are contaminated among other things withsodium sul fate and sodium chloride. Some waters also contain sodiumcarbonate or bicarbonate. In the use of these waters for boilers, theirevaporation in the production of steam causes concentration of theimpurities, and hence a gradually increasing sulfate and chloridecontent, and in the case of those containing sodium carbonate orbicarbonate, of carbonate and hydroxide content as well, the latterbeing due to the decomposition of sodium carbonate into caustic sodawhen in solution at boiler-water temperatures. In a water containingcalcium and sulfate, the concentration'of the latter will not becomegreat as evaporation proceeds, because the solubility of calcium sulfateis readlly exceeded, and it deposits as adherent scale on the heatingsurfaces, thus removing the sulfate.

In the treatment of waters to prevent formation of adherent scale, thechemicals used are, in general, lime and soda ash, soda ash alone,sodium chloride through the medium of the zeolites, or artificialproducts having their properties, or at high pressures, the phosphateradical either as phosphoric acid or one of its sodium salts. Whetherthe treatment is accomplished on the feed water and external to theboiler, or directly on the water in the boiler, the result is much thesame as regards the amount of soluble solids dissolved in the boilerwater. The removal of the scale forming constituents from the waterleaves therein soluble salts, such as sodium sulfate, chloride,carbonate, hydroxide, which increase in concentration as evaporationproceeds, and exercise their influence in retarding the disruption ofbubble-enveloping films in the steam drum. \Vhether the treatment isexternal or internal, suspended material of like type develops in theboiler water. As noted heretofore, the character of this material, andnot its quantity, is of greatest significance in its effect on foaming.

From the standpoint of prevention of ad herent scale, the concentrationsof carbon ate, phosphate and hydroxide which are essential in boilerwaters have been summarized in my Patents Nos. 1,613,656 and 1,613,701of January 11, 1927. Also there the relation of these concentrations tothe sulfate concentration in the boiler water, and the operatingpressure of the boiler have been described. And further therein, I havestipulated the concentrations of caustic alkali in the boiler waterwhich are satisfactorily inhibitive to corrosive action on the surfacesof the boiler, and have designated means whereby the content ofinsoluble solids or suspended matter in the boiler water may be reducedas desired.

Inasmuch as the conditions for the preven tion of adherent-scale growthand corrosion and the accumulation of suspended insoluble solids forhigh concentrations of soluble solids in the boiler water can be readilyand economically controlled, the function of the boiler blow-downbecomes that of so limiting the concentrations in the boiler water ofsodium sulfate, sodium chloride, saponitied material, etc, that thecoalescence and disruption of the steam-enveloping liquid filmsthroughout the boiler water and the steam-water interface shall occurwith a rapidity compatible with the delivery of steam of suitablequality through the steam outlet.

In some plants, the use of evaporated make-up water limits the solubleand insoluble impurities in the boiler water mainly to those derivedfrom condenser leakage, and in these plants, the amount of boilerblowdown requisite for satisfactory operation is very small. In manyplants, however, such evaporation is not economical, and a large make-upof natural or treated water must be used. If the natural water containsa relatively small amount of impurities, the blowdown may be onlynominal; but if it contains a large amount of impurities, the blowdownmay be excessive for the boiler to deliver a good quality of steam.

However, I have found that certain conditions may be maintained in theboiler water and at the steam-water interface of the steam drum. whichwill negate the retarding ett'ect of increasing concentrations ofsoluble and insoluble impurities in the boiler water on the coalescenceand disruption of the liquid films enveloping the bubbles of steam, andthus effect their rapid coalescence and disruption.

I have found that when the hydrogen ion concentration of a boiler wateris somewhat greater than that of pure water, so that, when cooled toroom temperature its pH value is -l or less, it may contain relativelylarge quantities of the impurities characteristically present in boilerwaters, and yet show no signs of foaming. But such a condition cannot bemaintained in a boiler water without damage to the boiler, inasmuch asat this concentration of hydrogen ion, the boiler metal corrodesrapidly. In fact, from the standpoint of corrosion, a slightly alkalineboiler water is better than a purely neutral water. Thus an alkalinityof approximately parts per million of hydroxide (OH) or more, the amountdepending mainly upon the concentration of sulfates and chlorides in theboiler water, is satisfactory.

Under these concentrations of alkalinity, slow coalescence of thebubble-enclosing liquid films readily occurs in boiler waters, causingfoaming. Experiments made by the boiling of water in open beakers orflasks will not show this effect readily, due to the presence of air atthe steam-water interface. The conditions in a boiler are different fromthose of the open beaker or flask, in that substantially pure air-freesteam is in contact with the surface of the water. Apparently thepresence of a gas in the steam above the water tends to cause disruptionof the bubbleenclosing liquid films, a condition which is not normallyencountered in steam boilers.

In tests made on boiler waters synthesized from distilled water and purechemicals, and on samples of boiler water obtained from boilersoperating under diverse conditions, I have found that we may dividethese waters into three classes according to the means which must beused to induce ready coalescence of disruption of the bubble-enclosingliquid films, as follows:

Class A consists of those boiler Waters which contain calcium, barium orstrontium salts, but only limited quantities of saponified material. Theminimum quantity of the metal radicals noted which must be present insolution is represented by the concentration of calcium present in awater fully treated by soda ash as defined in my Patent No. 1,613,701,or its chemically equivalent amount of barium or strontium. Thealkalinity of the water should be notless than 25 to 50 p. p. m. ofhydroxide. This class therefore includes all boiler waters derived fromwaters which contain sodium carbonate or bicarbo nate in their naturalstate, or treated by a zeolite or which are treated externally or internally by a hydroxide or carbonate as by lime, caustic soda, soda ash,barium hydroxide, barium carbonate or a combination thereof, unless theyare seriously contaminated by saponified material.

In a boiler water of this character, the maintenance therein of alimited concentration of certain ty ies of substai'ices hereafterspecified, or of the products derived therefrom by their chemicalreactions with the hot boiler water. results in ready coalescence ordisruption of the bubble-enclosing liquid films, even though thesulfate, chloride, hydroxide or carbonate, etc., concentration may reachrelatively high values. Thus, the amountof blowdown necessary whensatisfactory operation requires the maintenance of 2000 p. p. 111. onlyof sulfates or chlorides in the boiler water may be reduced to thatnecessary for the maintenance of 10,000 or 20,000 p. p. in. or more ofsulfates or chlorides.

One type of substance which effects this result satisfactorily istartaric acid (a dihydroxydiearboxylic acid) or the metallic salts oracid salts thereof. Another type of sul stance is citric acid (ahydroxytricarboxylic acid), and the metallic salts or acid saltsthereof. The effectiveness is not as great as that of the preceding typeof substance. A third type of substance is a trihydric phenol aspyrogallol; gallic acid, which on heating yields pyrogallol; and thetannins or tannic acids, which when boiled with dilute acids, assulfuric or hydrochloric, yield gallic acid, and which are regarded asprobably having the constitution of a peutadigalloylglucose. The tanninsare widely distributed in the vegetable kingdom, and vary to some extentin their properties, but have the common property of yielding gallicacid when treated as stated. The metallic or acid salts of gallic ortannic acids are likewise effective. This third type of substance is themost effective of all.

The term a tannin is intended to include the various substances havingthe properties of tannin and to include gallic acid, pyrogallol andsimilar substances.

All of these substances have the anti-foaming effect of the tannins.They are, so far as my present investigations haveshown, organic bodiesof the general type above set forth, and in general, appear to be thehydroxy acids of the aliphatic series and the dior tri-hydroxy acids ofthe aromatic series. These have an anti-foaming effect in connectionwith the hydroxyl radical and alkaline earth metal radicals, such ascalcium. Apparently, the antifoaming eflectof these substances isdependent upon the presence either naturally or by additions in thewater, of an alkaline earth metal, such as calcium, barium or strontium,together with hydroxyl. Tests have indicated that the presence of allthree of these substances namely, one of the antifoaming organicsubstances above mentioned, an alkaline earth metal and hydroxyl, aredesirable, if not requisite for the prevention of foaming.

It is infeasible to control by tests on the boiler water theconcentrations of these various organic anti-foaming substances in theboiler Water which should be maintained for optimum results, inasmuch asanalyses therefor are difficult, and also, at the temperature of theboiler water, decomposition may be occurring. A practical method formaintaining the effective concentrations in the boiler water is todissolve the material in the feed water to a concentration which isfixed by the concentration to be carried in the boiler water, and theamount of blowdown of the boiler. The concentration of the tartaric,

gallic or tannic radical to be carried in the boiler water may vary froma minimum of approximately 25 parts per million to a maximum of about100 parts per million, calculated on the basis of the substance asintroduced depending upon the per cent of rating at which the boiler isoperated, and the eoncentration of dissolved and suspended materials inthe boiler water. Thus. if a concentration of parts per million in theboiler water is advisable, and the boiler blowdown is 5% of the enteringfeed water, then the concentration of the tartaric, gallic or tannicradical in the entering feed water must be 4 parts per million; but ifthe blowdown is only 1%, then the concentration must be 0.8 parts permillion. The amount of blowdown is readily obtained from a comparison ofthe chloride or sulfate concentrations of the boiler and feed waterrespectively, if not known by direct measurement.

The effectiveness of these organic antifoaming substances in thepresence of the concentrations of calcium, barium or strontium andhydroxyl, above noted is satisfactory. Also, the effectiveness of thesesubstances does not diminish rapidly so that their introduction into theboiler water may be controlled as noted above. The control of thehydroxyl and alkaline earth metal concentration is effected as describedin my application Serial No. 718,322.

The requisite concentration of these materials in the boiler water forsatisfactory results may be readily judged by observations made on thetemperature of the steam leaving the superheater or on the water in thegauge glass of the boiler. Thus if the temperature of the superheatedsteam is subject to sudden drops, or the water level is fluctuatingviolently, boiling conditions are unsatisfactory, and a higherconcentration of the treating substances should be maintained. Or, ifthe amount of discharge from steam traps connected with steam purifiersattached to the steam outlet of the steam drum is excessive, theconcentration of the treating substances should be increased within thelimits heretofore defined.

If the addition of the treating substances within these limits does notproduce satisfactory boiling conditions, then the boiler must either beblown down, as the allowable limit of saponifiable material has beenoverstepped in the boiler water, and no excess of treating substance canbe effective; or a gas may be introduced in conjunction with thesetreating substances, as hereinafter described under Class C waters.

Class B consists of those boiler waters which contain less calcium,barium or strontium than that indicated as a minimum for the Class Awaters, and which contain only limited quantities of saponifiedmaterial. The waters of Class B, therefore, comprise those in which thecalcium content is very low as may be the case when the feed water isdistilled water, and the only contamination of the boiler water arisesfrom condenser leakage, or wet steam from the evaporator; or as occurswhen the final conditioning of the boiler water is accomplished by theuse of phosphate radical as described in my Patent- No. 1.61:3,656.

Under these conditions, the types of substances designated for the ClassA waters are very little, if at all, effective. On the other hand, thetypes of substances designated here inafter as effective in negatingretardation of coalescence or disruption of the liquid bubbleenclosingfilms for Class B waters, are likewise effective on Class A waters.

The fundamental substances which appear to be most effective on watersof Class B, are alcohols, particularly the monohydric alcohols. Theirsolubility in the boiler Water must be slight. As they are allsubstances which volatilize in the steam, they should either be suppliedcontinuously in small amount to insure their presence in the boilerwater, or should have a relatively low vapor tension at the temperatureof the boiler water. Some of the alcohols among others which have thesecharacteristics are cetyl, ceryl, and myricyl alcohol; and the sterolalcohols, as cholesterol and phytosterol, which are all monohydricalcohols. These substances are not readily synthesized in thelaboratory, nor do they occur in pure form in nature. Some of the morecommon materials from which they may be obtained by chemical treatmentare as follows:

Myricyl alcohol from beeswax (various varieties contain differentpercentages of this alcohol) and carnahuba wax.

Ceryl alcohol from Chinese insect wax, carnahuba wax and wool fat.

Cetyl alcohol from spermaceti, which also contains other effectivealcohols as dodecyl, tetradecyl and octadecyl.

Monohydric alcohols, as dodecyl, found in relative large percentage insperm oil and arctic sperm oil.

Cholesterol, found widely distributed in the animal kingdom, in smallpercentage; in higher percentage in gallstones, wool fat (wool wax orlanolin) and the shark oils; and in small percentage in butter fat, eggoil, cod liver oil, and others.

The vegetable sterols, as the isomeric phytosterols, widely distributedin the vegetable kingdom, and found in small amounts in various oilsderived therefrom, as cottonseed oil, corn oil, wheat oil, olive oil,and many others; also as a glucoside in olive bark and in nut-- meg,taraxacum root and bryony root, and many others.

The alcohol, which is the desirable and effective component in thesevarious natural substances, may occur uncombined, to some PUEMCH extent,as myricyl alcohol in carnauba wax, but in general is found as the esterof an or ganic acid. Thus the cetyl alcohol exists in spermaceti as theester of palmitic acid; myricyl alcohol in beeswax as the ester ofpalmitic acid and ceryl alcohol in Chinese insect wax as the ester ofcerotic acid.

If the natural substance, as beeswax, spermaceti, lanolin, orcarnaubawax, is introduced into a slightly alkaline boiler water whichis free from saponi tied material or contaminated therewith to a verylimited extent, it is elfective in speeding up the coalescence ordisruption of the liquid films, because, as the alkalinity of the boilerwater breaks up the esters into a salt of the acid and the uncombinedalcohol, the latter is free to exercise its effect on the films. Thesalts of many of the acids, however, as stearic, palmitic, oleic, etc.,are effective in stabilizing the films; and as the alcohol volatilizeswith the steam while the sodium palmitate or other organic salt remainsin the boiler water the final condition is worse than the initial.Finally, then, unless the blowdown is appropriate to avoid thecondition, the accumulation of the salt derived by saponification of theester becomes suificient to nullify completely the effectiveness of thealcohol. Also, the hydrocarbons contained to some extent by most of thenatural substances, and the saponification of the lycerides of the fattyacids therein seem to asten the stabilization of the films.

Thus the regular use of the natural substances herein described may bemade effective by their introduction into the boiler water continuouslyin small amount, in conjunction with an amount of blowdown suflicient toprevent too great accumulation of saponified material or hydrocarbonsderived from the substance. Under these conditions, the use of carnaubawax is preferable to the use of beeswax, lanolin, and shark oil, becausethe saponified material collects less rapidly in the boiler water. Theuse of carnauba wax is preferable to the use of spermaceti and sperm oilfor a like reason, and further because the alcohol of the former is lessreadily volatile in the steam than those from the latter two.

I prefer, however, to saponify the natural substance, and to extracttherefrom the alcohol or alcohols, and thus introduce them into theboiler water in a condition of commercial purity, thereby avoiding theaccumulation of saponified material.

The choice of alcohol to use is based on the operating pressure. Thus,the sterol alcohols are least volatile in the steam, and hence ofgreatest value at high operating pressures. At lower pressures, ceryl,cetyl, or myricyl alcohols may be entirely satisfactory, or even themore readily volatile alcohols from the sperm oils.

The amount of alcohol which much be continuously introduced into theboiler water depends upon the operating pressure of the boiler, thevolatility in the steam of the alcohol used, the per cent of rating atwhich the boiler is operated, the concentration and type of dissolvedinorganic salts and suspended matter in the boiler water, and thecontamination thereof with saponified material. The amount required maybe conveniently controlled by the smoothness of boiling as indicated byany sudden drops in the temperature of the superheated steam orfluctuations in the level of water in the gauge glass of the boiler, orby the amount of discharge from steam traps connected with steampurifiers attached to the steam outlet of the steam drum.

In the presence of saponified material, the substances specified underthe Class B waters may be used in conjunction with the introduction of agas, as hereinafter specified under Class C waters.

Class C consists of boiler waters which may conform to the conditionsspecified in either Class A or Class B as regards their concentration ofcalcium, strontium, or barium, but in which the concentration ofsaponified material is great enough to render unavailing without aid theeffectiveness of the substances specified in the two preceding classes.

Under these conditions, I have found that the continuous introductioninto the boiler water of a small amount of gas, as air, nitrogen, stackgas either without purification or purified of its sulfur dioxide andcarbon dioxide, or natural gas, is very effective in decreasing thestability of the liquid films and hence hastening their coalescence ordisruption. Especially is this true if the gas is introduced in the formof fine streams well distributed over and slightly below the steanrwaterinterface.

The nature of the gas used is immaterial, so long as it does not remainin solution in the boiler water.

The effect of the gas is apparently to cause a greater partial pressureon one side of the liquid films of the bubbles than on the other side,and consequently, to tend to cause disruption or instability of thefilm. If there be only pure steam on both sides of the film, theequilibrium pressure and temperature on the two sides is the same, butif there be a greater partial pressure of the gas on one side of thefilm, there will be a greater total pressure on that side of the film,which will tend to make it unstable.

The quantity of gas used must be based upon the per cent of rating atwhich the boiler is operated, the amount of saponified material in theboiler water, the concentration and type of soluble salts and suspendedmatter therein, and whether it is used in conjunction with the materialsspecified as efl'iaci u f r G a s A an C a s B waters, or

without their aid. The maintenance of a sufiicient supply of the gas isreadily recognized by observations of the temperature of the superheatedsteam, by sudden fluctuations in the level of water in the gauge glassof the boiler, or by the amount of discharge from steam traps connectedwith steam purifiers attached to the steam outlet of the steam drum.

This use of a gas to produce instability of the liquid films, and hencea more rapid coalescence and disruption of the films, is not limited inits application to boiler waters, but may be made use of in allprocesses of evaporation in which a slight dilference in the partialpressure of the components on the two sides of the liquid film rendersthat film unstable. The control by its use of the purity of condensatein an evaporation of waste liquors may be cited as an example.

In these specifications the terms boiler and boiler water are notlimited in their significance to the generation of steam at a pressuresuflicient for power purposes, but have rather the broader meaning ofthe boiling of a liquid in any closed vessel, as an evaporator.

In the practice of the invention, the use of the materials specifiedunder Class A in conjunction with those specified under Class B and/orClass C for waters of Class A may be found advisable at times. Likewisethe use of gas in conjunction with the materials specified under Class Bmay be advisable also for Class B waters.

Claims directed particularly to the use of an alcohol or wax arepresented in my application, Serial No. 283,456, filed June 6, 1928 as adivision of this case.

While I have specifically described the preferred embodiment of myinvention, it is to be understood that the invention is not so limited,but may be otherwise embodied Within the scope of the following claims.

I claim:

1. The process of inhibiting foaming in steam boilers, comprisingbubbling a small amount of insoluble gas through the steamwaterinterface.

2. The process of inhibiting foaming in steam boilers, comprisingmaintaining in the water a substance which accelerates the coalesenceand disruption of the liquid bubblefilms and introducing a small amountof insoluble gas into the water.

In testimony whereof I have hereunto set my hand.

RALPH E. HALL.

